Posts Tagged ‘soybean’

The lower than normal rainfall observed has limited spread of soybean rust throughout the Southeast and South-central U.S. Soybean rust was recently identified in kudzu in Leon County, Florida on April 17. However, at this early point of the growing season, and with minimal infection across the South, the risk of long distance soybean rust spread to the north is minimal. The map below shows areas currently positive for soybean rust in northeastern Mexico and throughout the Gulf Coast area of the United States. A relatively mild winter likely contributed to more conducive overwintering conditions for rust on kudzu along the Gulf Coast. A drier last 30 days across the Southcentral along with drought conditions across the Southeast has made further rust spread unlikely in these areas.

I have had a number of calls about burndowns not being effective for no-till soybeans. Poor control can be attributed to a number of reasons, weeds were too large, gallonage was too low, and wrong products or adjuvants were used. But the question is what to do now. First determine what was not controlled. In most cases it is marestail or horseweed, and it needs to be controlled before you plant because there are not effective postemergence herbicides for it (unless you are using Liberty Link soybeans and use Liberty 280). It’s too late to rely on 2,4-D to control marestail because you need the 1 qt rate to provide effective control. The 1 qt rate of most 2,4-D products require 4 weeks before planting and in too many places sensitive plants have emerged. If you are on an appropriate soil type, Sharpen, with all the required adjuvants, is an option (see the label). Sharpen is not an option for coarse-textured soils because it also needs 4 weeks between application and soybean planting. Liberty or Ignite can be used, but it works best on days with full sun shine and requires excellent coverage (at least 20 g/A and medium droplet size). A chlorimuron-based herbicide is another option, but you need to use rates that will provide good suppression/control. Chlorimuron rates equivalent to 1.5 oz of Classic is needed (see table below). In most situations, the chlorimuron-based products should be used with a burndown herbicide (glyphosate or Gramoxone) and refer to their label for adjuvants.

Fields that will be planted to no-till corn or soybeans may have excess weed growth due to the warm winter. This will make burndown treatments more challenging, and in some cases it is unrealistic to expect complete control with only one application. In those cases, you may need an application now, followed by an additional application at planting. For weeds that are hard to kill with glyphosate, additional herbicides such as 2,4-D can enhanced the control (for instance, mustards); while other herbicide combinations can reduce glyphosate control (for instance, atrazine in combination with glyphosate for ryegrass control). Be sure to assess each field, and determine the best approach. Do not assume you can spray a week ahead of planting and achieve a clean seedbed to plant into.

While we are a long way from dealing with soybean rust (SBR), it is still a major threat to soybean production in the South. The incidence of SBR in the last two years has been low to almost non-existent (last season). However 2012 is looking different possibly. We have had a record warm winter which extended to the deep South as well where soybean rust has to overwinter on other live hosts, primarily kudzu. In 2012 soybean rust has been found in Cameron County, TX, Baldwin County, AL, Iberia Parrish, LA and now the most recent find has been in Gadsden County, FL. This is the northern panhandle region where most of the soybeans in FL are grown. The Florida find is the earliest (March 22) that soybean rust has been detected on kudzu in FL. Besides the warm weather, they have had much more rain than in previous years, which has also favored the fungus. The panhandle of FL has only had 70 hours below freezing all winter compared to most years when they see more than 200 hours each winter. The early season is promoting early kudzu growth which could mean increasing levels of rust on kudzu that moves to soybeans that produces spores that have the potential to blow north in storm fronts, tropical storms and hurricanes.

I am not predicting anything here, because it is way too early, but the scenario is developing that we have the potential for soybean rust increasing after record low levels of the past two years. One year the South had a late freeze that killed the kudzu and the fungus along with it, which greatly slowed and reduced soybean rust that year. You can’t predict what will happen, but the forecasters do not forsee that happening this year. If you want to keep up with soybean rust development the ipmPIPE (http://sbrusa.net) is continuing this year despite reduced federal funding Thanks to industry partners, the website will continue for 2012.

This is not a good time to take nematode samples for root-knot nematode detection but if soybean cyst nematode (SCN) is a concern and you are planting soybeans this spring it is not too late to sample. SCN is detectable all season long as long as the ground is not frozen or flooded. Remember that snap beans are also a host of SCN and fields planted to snap beans should be checked for SCN. Lima beans are resistant to SCN. Testing most of the major baby lima bean cultivars confirmed that they are resistant to the major race or genotype of SCN that is prevalent in Delaware.

Nematode sample bags are available at all the county Extension offices as well as the information sheet which needs to accompany each sample. This info is also available online at http://ag.udel.edu/extension/pdc/. Samples do not have to be submitted in these bags but are there for your convenience. Soil samples for nematode detection should be at least 2 cups of soil placed in a Ziploc plastic bag. Do not use paper bags unless they are double-bagged with a plastic bag.

Soybean Vein Necrosis VirusI just wanted to add a brief summary for our WCU readers that soybean vein necrosis virus has been seen in all three counties in Delaware and apparently is widespread in the surrounding states of PA, MD, and VA. We see it everywhere we look now but I am not sure how much effect it is having on yield at this point. Researchers in other parts of the country are also working on it and have a few more years experience with it. It is premature to say too much about it other than we now know what is causing the symptoms we have seen this year and probably last year as well, but did not know what it was. By the time the winter meetings occur hopefully I will be able to share more hard facts about the disease and control options for our region.

Early symptoms of soybean vein necrosis virus (SVNV) from DE

Soybean Cyst Nematode
Do not ignore soybean cyst nematode. Soil sampling after harvest before any fall tillage is recommended for fields to be planted next season to soybeans following this year’s crop. Do not plant SCN susceptible varieties without soil testing first. Soil sample bags and information sheets are available from the county Extension offices for $10/ sample bag.

Soybean Rust Risk Assessment (ZedX, Inc. & PSU)Despite recent heavy rainfall along the Atlantic Coast and throughout the Northeast from the remnants of Lee and Hurricane Irene, source inoculum in the Southeast was likely still too low to cause widespread transport and deposition of spores further north. Soybean rust was, however, identified in extreme southwestern Georgia for the first time this season. Due to the ongoing drought in Texas and Oklahoma, the slow progression this season in the Southeast, and the fact that the primary soybean production season is in the later stages, it is unlikely that soybean rust will spread as far as it has in years past. As such, the risk area will remain rather minimal in spacial coverage (see map below). Double crop soybeans along the Gulf Coast could still be at risk for soybean rust as the season progresses, but even double cropped soybeans are at a minimal risk.

The above image displays the current threat level of soybean rust. The yellow “wait” areas are considered slightly at risk, orange “watch” areas are at moderate risk, and red “warn” areas are at great risk or already identified positive for soybean rust. Risk areas are estimated based on meteorological factors affecting spore transport and deposition and factors conducive for further development within the canopy such as temperature and moisture. Biological factors such as host plant and crop phenology are also considered. Risk assessment maps are produced by the PSU Ensemble Field Crop Rust Forecasting Program.

Soybean vein necrosis virus was confirmed this week by Dr. Yannis Tzanetakis, Department of Plant Pathology at the University of Arkansas. So the symptoms that we have been seeing and sharing with concerned growers are due to this new virus disease. Although I say it is new, some of the first symptoms were seen in 2008 in Tennessee and Arkansas. The subsequent work by the researchers at Arkansas discovered the new virus. They were able to report that this new virus disease, soybean vein necrosis virus (SVNV), belongs to a group of thrips-transmitted viruses. This group called the tospo viruses includes several that we see in greenhouse production in the region and in the field occasionally, namely tomato spotted wilt virus and impatiens necrotic spot virus.

Early symptoms of soybean vein necrosis virus (SVNV) from DE

This group of viruses are acquired by thrips feeding on infected plants and once acquired then the virus can be transmitted by the thrips for the rest of its life. We call this a persistent virus in the vector and this may allow us to control SVNV by controlling thrips. This is very preliminary and much work is being done in the Midwest to identify the thrips vectors and possible other hosts of the virus that may harbor it and allow thrips feeding to move it to soybeans. It is too early to know if that strategy will work practically. The question we all have is: will it reduce yield here or affect seed quality? So far I have not seen enough leaf loss to imply that some yield effects are possible. But we have some time to go before maturity, so the jury is still out on the yield effects here in the Mid-Atlantic. Fortunately this group of viruses is not known to be seed-transmitted and that is being addressed by these researchers.

The researchers from Arkansas have noted that often a single virus infection may not have much of an effect but multiple infections with other viruses may increase yield loss potential. We have occasional outbreaks of bean pod mottle virus and we have seen soybean mosaic virus and peanut stunt virus in soybeans in the region, so the potential is here for multiple infections. We do not have much information about the extent of other virus diseases in soybeans.

The other avenue of control is identifying sources of resistance and evaluating current soybean varieties for resistance. Ideally identifying sources of genetic resistance that are incorporated into good varieties will be best control strategy. That work is ongoing as well. It is too early to be making recommendations but growers need to be aware of this disease and know that work is being conducted to answer some of these pressing questions.

Also, do not ignore soybean cyst nematode. Soil sampling after harvest before any fall tillage is recommended for fields to be planted next season to soybeans following this year’s crop. Do not plant SCN susceptible varieties without soil testing first. Soil sample bags are available from the county Extension offices for $10/ sample bag.

The question has come up after the tremendous rainfall and damage caused by hurricane Irene whether or not one should spray their soybeans with fungicides. There was also a question just before the hurricane as to whether or not it was advisable to throw in a fungicide since many fields were to be sprayed for insects like podworm and stinkbugs. The answer to both those questions is highly dependent on each individual situation and a blanket recommendation cannot be made, especially because soybean rust is not going to be an issue this season.

Before we discuss the factors that play a part in determining fungicide need let me briefly explain why we rely on statistics and statistically valid research to answer these questions. Most of you are familiar with yield monitors and their output either because you have equipped your combine with one, or the custom combining operation that does the harvesting has one, or you have seen sample maps in a trade publication. If you haven’t then I’m sure you are still aware of the issue of variability in a field because of all the hoopla associated with any precision Ag equipment article or ad. These have been invaluable tools as they have really shown that an apparently uniform 40 acres is really quite variable. The obvious reasons for the variability are variations in soil type, depth, moisture and fertility just to name a few. But there are also many other less obvious reasons associated with pests since they are not uniformly distributed across any field as well. Some fields are more variable than others but every field is to some degree not uniform. The fact is if you took any production field and arbitrarily split it in half the average yield in each half would not be identical. In fact I would suspect that the vast majority in our area would easily have about 3-5 bu/A yield difference between each half. Again some might have greater uniformity and a smaller difference and some might have more. It is this variability that illustrates the need for statistics. Think of it this way. If the same field that we split arbitrarily in half produces yields that are different by 3-5 bu/A without a treatment then how do we know if we put a fungicide (or anything else) on half that it did anything? We use statistical tools and design experiments following certain statistical rules to answer that question. Two of the most important rules are replication and randomization. We will replicate so that we test a treatment more than once and get a measure of the variability in the response. We will randomize where each replicate of each treatment goes in the field to make sure they all have an equal chance of being in a high yield spot or low yield spot in a field. By using these and other tools we can then use the variability that is there to say whether or not a treatment has a certain probability of having an effect. The point is a bigger number doesn’t always mean that the treatment has a real or significant effect for those conditions. Sometimes a 3-5 bu yield difference could just be chalked up to being part of the background variation.

Ok, getting back to should we spray fungicides on soybeans after this heavy rainfall and wind damage. Let’s start with the damage. There are have been no independent scientifically sound experiments that show a consistent and reliable yield advantage to spraying fungicides in the absence of diseases. Physical damage by itself is not going to be corrected by a fungicide. Some fungicides reportedly have a positive effect on plants by counteracting components of plant stress responses. However, there is no proof from independently conducted field trials that this affect is large enough to affect yields in the field in non-diseased plants.

Stands that are lodged, leaning or matted down because of high winds tend to favor the development of certain diseases because the canopy remains closed and therefore remains wet for longer periods. Also, the distance a fungus needs to move either from leaf to leaf or soil to leaf in lodged or leaning stands is generally less. Heavy rains play a more important role than the stand damage in driving disease development. The rain from Irene provided two main avenues for greater disease development: spread of disease-causing agents if they were present; and a wet or humid environment that favors pathogen growth and infection of plants. A key that cannot be ignored is there has to be a pathogen around and in a form capable of being moved.

There are soybean diseases that will not respond to foliar fungicides. Bacterial blight and Bacterial pustule are diseases that are favored by hard driving rains. They look very similar to several fungal diseases, but because bacteria cause these fungicides cannot control them. Don’t get talked into spraying just because there are some leaf spots. There are also some fungal diseases that will not respond to foliar applied fungicides. The stem and root diseases in particular, Charcoal rot, Fusarium wilt, SDS and Phytopthora all are fungal diseases that cannot be controlled either preventatively or curatively at this time of year. One on-line source of brief disease descriptions with pictures is http://cipm.ncsu.edu/ent/ssdw/soyatlas.htm .

The soybean diseases that can occur in Maryland that can have an impact on yield, and can be managed with foliar applied fungicides are primarily: soybean rust, frogeye leaf spot and brown spot. Soybean rust due to the earlier drought in the South is not likely to be a problem this year. Its reestablishment and increase on kudzu has been severely delayed and is at the lowest level of infestation in August since monitoring for this disease began in 2005. Hurricane Irene therefore was highly unlikely to pick up enough spores from Florida and deposit them in Maryland to be an issue. Of the remaining two diseases Frogeye appears to have more potential for damage but is fortunately less common than Brown spot. Brown spot is the most common foliar disease but tends to be a lower canopy problem. Both are much more likely to develop if the previous crop was soybeans. I cannot find any data that would indicate there is much of a chance of getting a yield response from a fungicide by controlling frogeye or brown spot when the fungicide is applied after the R3 growth stage. The reproductive growth stages are: R1 initial flowering, R2 full flower, R3 small pods are formed at upper nodes, R4 full pod, R5 beginning seed, and R6 full seed. One on-line source for soybean growth stages can be found at: http://extension.agron.iastate.edu/soybean/production_growthstages.html . There are other common diseases that tend to form late in the season and tend to be considered seed quality issues. Another way to understand these late season diseases is that fungicides reduce them but without a significant change in yield. Only seed quality is improved. Although seed size is often improved, only rarely does that translate to significant yield increases. The most common of these diseases are purple seed stain, pod and stem blight, and anthracnose. There is a third category of generally late appearing diseases that are reduced by fungicides but we have little evidence that they respond to the fungicide with a yield increase, for example Phyllosticta leaf spot. In short as a general rule it takes a fair amount of foliar disease that starts early in the reproductive period, if not before, to cause enough damage to reduce yields that can be countered with a fungicide.

With regard then to our current post-Irene situation, here are my thoughts on fungicides in soybeans.

● Scout first. If you know there is a specific problem that is starting then you can much more easily decide on the need for a fungicide.

● If you are a seed producer and the soybeans are between R1-R3 then consider spraying. You will protect some seed quality and have the potential for managing brown spot or frogeye if present and in that case may see a yield benefit. If the beans are between R4 and R5 then consider spraying if you haven’t sprayed earlier. However, expect only a seed quality improvement.

● If you are growing soybeans for general production and are between R1-R3. Scout first! If you have symptoms of frogeye leaf spot or of brown spot in the middle of the canopy then a fungicide application may be warranted. This is much more likely to occur if the soybeans were planted no-till into soybean stubble. If the field is at R4 and symptoms of frogeye or brown spot are evident then spraying is much less likely to improve yields. If the soybeans are at R5 then there is little evidence that you will get your money back from a fungicide application especially if it is just brown spot. If there is frogeye, it’s R5, and it continues to be wet the disease will get worse but there is no evidence that you will get a return on the fungicide. If you are not comfortable with not spraying then consider spraying in strips. In essence run your own test. It will cost you less and you will have a better basis for making the decision next time.

● Knowing the field history and rotation history are helpful. If you have soybean planted no-till into soybean stubble (it could be wheat-bean or barley-bean stubble) then you have the best chance of having diseases that can be reduced with fungicides and therefore get a yield response with a fungicide. The next level of risk based on crop rotation is when soybeans are just a year apart in the rotation, e.g., full-season beans followed by corn followed by wheat or barley and then double-crop beans. The lowest level of risk is there is a two-year or more rotation to soybeans. The most likely thing to happen with a fungicide application at R3 to R4 is improvement of seed quality (for example reduced purple seed stain or improved test weight) but not necessarily improve yields. The higher the risk based on rotation then the higher the chance a yield improvement will be attained. In my opinion, you have a 30-50% chance of getting a yield boost (3-5 bu/A) in soybeans with the higher chance coming from the higher risk situation and spraying at R3.

● Last point. Spraying for yield protection without knowledge of what is out there has already led to the development of a strain of frogeye that is resistant to strobilurins (Headline, Quadris, etc.). This has been confirmed in IL, KY and TN. It’s not a very good insurance program to spray against diseases when you just spray for yield boost because you are destroying one of the best tools you might need later on.

We have seen samples from DE and MD with unusual leaf spot symptoms that are often limited to the veins. There is a yellowing and reddening of the tissue and veins associated with the virus infection. The symptoms can progress to a necrosis of the tissue around the veins as well. This virus disease, soybean vein necrosis virus, once it has been confirmed, would be new and a first report for our area. It has been seen in parts of the Midwest and confirmed by researchers at the University of Arkansas. They have seen pictures of the samples we have received and are sure it is SVNV. If you have seen these symptoms on soybeans we would like to know about it, and receive some leaf samples. There are differences in susceptibility to this new virus among varieties so we would need to know what variety you are growing if you see these symptoms. There may be some confusion about these symptoms because they can resemble Cercospora leaf blight caused by Cercospora kikuchii, which also causes purple seed stain.

Sudden death syndrome was diagnosed on soybean from Sussex County. This was an unexpected find since I associate SDS with cool, wet seasons. The only control for SDS is to plant resistant varieties, or, if susceptible soybeans are to be planted, double crop soybeans are rarely infected. I would not expect this to be widespread but we did see it so be on the lookout for symptoms.

Foliar symptoms of SDS

Infected root on the left with diseased leaf compared to healthy stem and leaf.

Charcoal rot was also confirmed on soybean as well as corn. Soybeans are killed by the fungus resulting in dead, stunted plants in irregular patterns in the field. Symptoms are worse on full season early maturing varieties (Maturity Group III and possibly early Group IV’s). The diagnostic symptoms are the ashy gray stems and lack of roots. Splitting the lower taproot and stem longitudinally with a knife reveal the best diagnostic feature which is the presence of the small black microsclerotia in the pith and often the cortex. Charcoal rot is favored by hot, dry weather and is common in Delaware soils that have been heavily cropped to soybeans over the years. The tiny sclerotia overwinter in the soil and it just takes the right environmental conditions for the disease to occur. There is no usable resistance in soybeans to charcoal rot. The stress during flowering and early pod set triggers development of symptoms if that coincides with hot, dry, stressful weather. Most initial infections occur during the seedling stage and remain latent until flowering and later. Later maturing soybeans typically flower and set pods most years when conditions are less stressful and escape the yield reducing effects of the infection. Double crop soybeans are rarely affected by charcoal rot here in DE. Two year rotation will help reduce microsclerotia levels in the soil but if corn is used at least three years is suggested. Longer rotations would be better if they could be implemented.

Split soybean stem showing the charcoal gray discoloration in the pith with microsclerotia.

Close up of gray pith from the presence of the tiny microsclerotia of the fungus that causes charcoal rot.